In pulp industry, recovery furnaces are used as a chemical reactor and for the production of steam for internal use, for generation of electricity, and for sale. As the recovery furnace operates as a chemical reactor, the combustion conditions differ from those of an ordinary boiler, in that the heating surfaces of the furnace get covered extremely rapidly with combustion deposits, i.e. carryover/slag, dust and/or soot, which decrease the efficiency of the recovery furnace, particularly by reducing heat transfer in the furnace. In addition to soot, the flue gases contain inorganic chemicals, which condense on the heating surfaces of the recovery furnace.
In power boilers the thermal and chemically efficiency is normally depending on the mixture of fuel, combustible gases and the air in the furnace. In larger furnaces, there are local variations of the combustion depending on the location in the boiler. The combustion characteristics can for instance vary considerably between the wall and the middle of the furnace. An increased knowledge of the gas content and flue gas temperature in different furnace zones makes it possible to control the burning conditions to a greater extent in order to obtain an overall high combustion efficiency in the furnace, thus improving the use of heat surfaces and minimizing the emissions from the furnace.
Boiler furnaces require frequent cleaning of the heating surfaces by means of special cleaning apparatus, called sootblowers. Generally, the sootblowing system comprises about 10-80 sootblowers. The sootblowers clean the heating surfaces with high pressure steam, and generally about 2-10% of the steam production of the furnace is used for cleaning the furnace. If the time between successive cleanings in the furnace is too long, the dust-like particles get harder and/or sinter, and the deposits will be harder to remove. Thus, by minimizing the carryover in the furnace it is possible to also minimize the need for sootblowing and/or increase the efficiency of the production.
In order to control the chemical process and combustion process inside the furnace and to keep the sootblowing to a minimum, while at the same time cleaning sufficiently for the furnace to work efficiently, continuous and reliable measurements of the process are needed. However, to achieve the desired results is difficult due to the extreme temperatures and chemical conditions in the furnace and the fact that any sensors provided inside the furnace would themselves have to be cleaned from the soot or sintered dust from the process.
US2006005786 (Habib et al.) discloses a sootblower that is used inside a furnace. In order to control the operation of the sootblower, sensors are used to measure the properties of substances inside a combustion chamber connected to said sootblower. However, the technology does not disclose a method or device for measuring the conditions inside the furnace itself, and therefore does not present a reliable solution to the problem of monitoring or controlling the operation of said furnace.
The Japanese document JP63163124 shows the measuring of radiation energy inside a recovery furnace by providing a radiation thermometer on the wall surface of the furnace. Another method for measurement is shown in JP234185, where an optical fiber is inserted into a furnace to direct light from the process to a spectroscope for performing spectral analyses, and the European patent EP0947625A1 shows a method for measuring the conditions inside a recovery furnace by using a spectrometer for creating a continuous electromagnetic spectrum.
Another method is proposed by WO2004005834 (Schwade et al.), where a number of sensors and cameras are used to measure and monitor the conditions inside a furnace. The sensors are, however, placed inside the furnace itself, and so are themselves subject to the extreme conditions mentioned above. This severely limits the types of sensors that can be used, as well as the data that can be retrieved from them, and does not allow for detailed monitoring and control over the process inside the furnace.
These methods therefore all suffer from the lack of accuracy which arises when sensors are present in the highly chemical environment of the recovery furnace. Sensors mounted on motorized lances that are inserted into the furnace require cooling in order to preserve their ability to operate. They are also expensive due to the need of machinery that handles large probes of lengths around 4-8 m.
Inside the furnace a great amount of opaque flue gas obstructs the view, rendering it impossible to use ordinary measuring instruments to measure anything but the band of flue gas close to the wall of the furnace. Thus, no detailed information of the conditions towards the middle of the furnace can be achieved. Yet measurements must be made continuously during the process in order to control the operation of the furnace and initiate cleaning procedures when needed. The need for more accurate measurements is therefore apparent.